There are a number of natural isotopes of lithium (Li), in which the abundances of 7Li and 6Li are the highest, which are 92.48% and 7.52%, respectively. The two isotopes play important roles in nuclear material field; nevertheless, their nuclear reaction properties are quite different. 7Li is an indispensible molten salt coolant in thorium-based molten salt reactors. Since the thermal neutron absorption cross section of 6Li is quite high (up to 941 barns), while that of 7Li is only 0.033 barns, the molten salt reactors require >99.995% abundance of 7Li. Meanwhile, highly purified 7Li is usually used to adjust the pH value of primary coolant in pressurized-water reactors; and it is also used as a heat-conducting heat carrier agent in fusion reactors. Furthermore, the tritium which is of quite low abundance in nature can be produced by using neutron to irradiate 6Li (n, α) T nuclear reaction. 6Li is a fuel in nuclear fusion reactors, wherein the abundance of 6Li should be >30%.
Either thorium-based molten salt reactors or nuclear fusion reactors will provide a solution for the development of strategic new energies in China. Therefore, lithium isotopes being an indispensable strategic material, the development of processes for separating lithium isotopes and the development and manufacture of new lithium isotope extractants have always been the research hotspots, while at the same time they are also the technological obstacles of the field.
Currently, existing methods for separating lithium isotopes comprise electromagnetic method, molecular distillation, electro-migration, electrolysis and various kinds of chemical exchange methods (Xiaoan Xiao et al, Journal of Nuclear and Radiochemistry, 1991, 13, 1).
So far, most of the lithium isotope separating methods only remain in the laboratory research stage (Yaohuan Chen, Chinese Journal of Rarematerials, 1983, 2, 79). For example, neutral solvent extraction system (e.g. isoamylol/LiBr system), ion exchange system (e.g. hexanoic acid/kerosene system), chelating system (e.g. sudan I-TOPO system), etc., all have a comparatively low separation coefficient α (usually <1.010), and therefore cannot be used in industrialized extraction processes (Yaohuan Chen, Atomic Energy Science and Technology, 1987, 21, 433). The extractant reported in CN201110425430.4 has a low extraction rate (the one-time extraction rate is only 16%), and the hydrophilic ionic liquids are expensive and difficult to be recycled through phase inversion; furthermore, the extractant has not be used in multiple stage enrichment and separation experiments. The systems like crown ether and cryptand extraction systems enriched 6Li in organic phase. Although the separation coefficient α is comparatively high, the system is difficult to synthesize, has a high cost and high toxicity, and it also did not accomplish multiple stage extraction to enrich lithium isotopes (Yanlin Jiang et al, Atomic Energy Science and Technology, 1986, 20, 1).
At present, lithium amalgam chemical exchange method can satisfy the technical requirements for isotope separation, and has become the only method for industrial production of lithium isotopes (Palko, A, A, et al. J. Chem. Phys, 1976, 64, 1828). However, lithium amalgam method requires a large amount of mercury, which is severely harmful to operators as well as the environment. Moreover, since lithium amalgam method is easy to enrich 6Li, and lithium amalgam will gradually decompose in the extraction column, it is not suitable for the separation processes which have multiple stages and require high 7Li abundance (>99.99%).
Therefore, there is an urgent need in the art for an extractant which is safe, environment friendly, efficient, suitable for multiple stage enrichment and separation, and easy to enrich 7Li.